1. Field of the Invention
The present invention relates to improved morphology discrimination (MD) for sensing irregular cardiac rhythms or arrhythmias in an implantable cardioverter defibrillator (ICD).
2. Related Art
ICDs are medical devices that may be surgically implanted in a patient to monitor the patient's cardiac activity and to provide electrical stimulation in order to correct irregular cardiac rhythms (i.e., arrhythmias). The occurrence of the arrhythmias can be attributed to anomalies in the heart's electrical conduction system.
Arrhythmias can generally be thought of as disturbances of the normal rhythm of the heartbeat and can be divided into two major categories: bradyarrhythmia and tachyarrhythmia. Tachyarrhythmia is an abnormally rapid heart rate (e.g., over 100 beats per minute, at rest), while a bradyarrhythmia is an abnormally slow heart rate (e.g., less than 50 beats per minute).
Tachyarrhythmias can be further subdivided into tachycardia and fibrillation. Tachycardia is a condition in which the electrical activity and rhythms of the heart rate are rapid, but organized. Fibrillation, on the other hand, is a condition in which the electrical activity and rhythm of the heart are rapid, chaotic, and disorganized. Tachycardia and fibrillation are further classified according to their location within the heart, namely either ventricular or atrial (supra-ventricular).
A depolarization signal, which is a small electrical impulse, triggers contraction of myocardial tissue within the human heart, causing it to beat. Depolarization signals that correspond to the contraction of the atria are referred to as P-waves, and signals corresponding to the contraction of the ventricles are referred to as R-waves. The complex of depolarization signals produced by a normal heartbeat is commonly referred to as a QRS complex. The sequence of QRS complexes produced by a beating heart creates an electrogram (EGM) or electrocardiogram (ECG) signal (depending upon whether the signal is detected within the heart or on the patient's skin, respectively.) The invention described herein is directed to evaluation of EGMs that can be monitored by appropriate electrical circuitry to determine the condition of the heart. It will be understood by those of skill in the art that the invention is not limited to the location of the sensing electrodes.
A commonly used technique to help discriminate between supra-ventricular and ventricular tachyarrhythmias is morphology discrimination (“MD”). Such a technique is described in U.S. Pat. No. 5,779,645 to Olson et al., which patent is incorporated herein by reference in its entirety. MD techniques use dynamic template matching in order to classify and distinguish complexes associated with supra-ventricular and ventricular tachyarrhythmias. The MD techniques enable an ICD to examine a number of intracardiac electrogram complex characteristics such as amplitude, polarity, and shape, that might be representative of a fast heart rate. The morphology discrimination algorithm then compares each complex with a patient specific template that includes complexes representative of the patient's normal cardiac activity.
Morphology discrimination techniques can operate on the basis of an “X out of Y” system with regard to analyzing a sequence of complexes following detection of a tachycardia. The Y component is representative of the total number of complexes within the sequence, and the X component is representative of the total number of matches out of Y number of complexes necessary to determine that the rhythm is supra-ventricular in origin. A comparison between sensed complexes and the patient specific template is used to determine match or non-match.
In some instances, however, certain cardiac conditions can trigger unacceptably high error rates in classifying arrhythmias as ventricular or supra-ventricular. These errors, due to false classifications, ultimately trigger unnecessary and painful electrical shocks to a patient's heart by the ICD.
The false classification issue was documented in the following abstract: F. Duru, C. Scharf, and C. Brunckhorst et al., “MD Feature in ICDs During Rapid Atrial Pacing and Atrial Fibrillation,” Europace 2001; 2: B77 (Abstract 926). This was an acute study involving a series of 20 patients implanted with ICDs evaluated prior to discharge with an electrophysiology study. During that study, incremental atrial pacing was performed at rates of 100, 120, and 140 beats per minute (BPM). Atrial pacing was accomplished via a separate temporary lead inserted into the atrium so that it would not be part of the implanted device. Using the same temporary lead, atrial fibrillation was induced in 17 of the patients.
In this study, the morphology discrimination criteria were set to a 60% match for each complex with a requirement that at least 5 of 8 complexes score greater than 60%. A satisfactory template could be obtained in all patients with MD scores greater than 90% during rest. During incremental rapid atrial pacing, the morphology discrimination feature alone correctly identified the result in ventricular rhythms as being supra-ventricular in 16 of 19 patients for a specificity of 84%. In the group with atrial fibrillation induced, 14 of the 17 were correctly identified as supra-ventricular in origin. In addition, there were isolated beats in four other patients that scored less than a 60% match. Based on the examples included, there were isolated beats with a rate-dependent ventricular aberrant conduction, particularly on the shorter cycles, that accounted for most of the failures to identify, the rhythm as supra-ventricular. There was one example, however, where a commonly used “eyeball” technique failed to demonstrate a discernable difference between the “normal beats” with most of the complexes being scored less than 50%.
Rate-dependent bundle branch block is the most common etiology of the failure of MD techniques to recognize supra-ventricular tachycardias. Failure to recognize supra-ventricular tachycardias is normal behavior of the MD techniques because rate-dependent bundle branch block changes the morphology of the complex from that of the baseline template. There is not a single discriminator that will work in every case.
Other approaches for distinguishing between ventricular and supra-ventricular tachyarrhythmias include techniques such as “sudden onset” and “interval stability.” Sudden onset includes monitoring EGM complexes to determine a rate of change the intervals associated with cardiac complexes. For example, when a person exercises, their heart rate typically increases relatively slowly, representing a correspondingly slow rate of change in the associated intervals. During a ventricular episode, however, changes in the heart rate occur much more abruptly. The sudden onset technique attempts to use this abruptness as a characteristic to distinguish ventricular episodes from supra-ventricular episodes.
Interval stability entails monitoring and analyzing ventricular to ventricular (R—R) intervals. Although morphology discrimination, sudden onset, and interval stability are somewhat successful in distinguishing between ventricular and supra-ventricular tachyarrhythmias, more reliable techniques are needed.
By way of background, the conduction system of the heart includes an AV node, which serves as a buffer between the atrium (upper chamber) and the ventricle (lower chamber) of the heart. Specifically, the AV node controls the transmission of electrical impulses (originating in the upper chamber) from the upper chamber to the lower chamber. There are some rhythms in the upper chambers that are inherently fast. And while these fast rhythms, or high atrial rates, are not consistent with optimized cardiac performance, they are not lethal.
During cases of high atrial rates, the buffering provided by the AV node prevents some of the electrical impulses in the atrium from reaching the ventricle such that although the atrium might be beating rapidly, the ventricle might not beat quite as rapidly. Although the buffering provided by the AV node prevents transmission of most of the electrical impulses from the atrium to the ventricle, some impulses actually get through, making the ventricular rhythms appear faster. Fast rhythms in the lower chambers of the heart, however, can be lethal. Therefore, it is important to know whether any fast ventricular rhythms actually originate in the ventricle, or are caused by rhythms that originated in the atria.
One way the AV node provides buffering is that it slows down some fast-rate impulses while causing others to completely drop out. In some cases, these fast supra-ventricular rhythms create corresponding changes in the rates of the ventricular rhythms. At the same time, however, these fast rate supra-ventricular impulses cause the morphology of the associated QRS complexes to resemble normal complexes. Therefore, in these cases, the supra-ventricular rhythms can be effectively distinguished from the ventricular rhythms using morphology discrimination. In other cases, however, the supra-ventricular rhythms create distortion in the morphology of the ventricular QRS complexes, creating errors in the morphology discrimination analysis. One of the conditions that triggers these errors is known as variable conduction. It has also been noted that these variable conduction supra-ventricular rhythms are typically characterized by short R—R intervals, which create the mis-match between the ventricular QRS complexes and the sinus template.
What is needed therefore is an improved morphology discrimination technique to more accurately classify supra-ventricular complexes created due to conditions such as variable conduction.